FIG. 9 is a general laser machining device for hole drilling. In FIG. 9, a laser machining device 101 has a laser oscillator 103 for generating a laser beam 102, a bend mirror 104 arranged to guide the laser beam 102 emitted from the laser oscillator 103 in a desired direction by reflection, galvanometer scanners 106a and 106b respectively having galvanometer mirrors 105a and 105b serving as movable mirrors sequentially arranged along an optical path, an Fθ lens 108 for converging the laser beam 102 the traveling direction of which is controlled by the galvanometer scanners 106a and 106b onto an object 107, and an X-Y stage 109 driven on an X-Y plane and having an upper surface on which the object 107 is fixed.
The operations of the respective components used when hole drilling is performed by using such a laser machining device will be described below.
The laser beam 102 having a pulse waveform oscillated depending on a frequency and an output value which are predetermined by the laser oscillator 103 is guided to the galvanometer scanners 106a and 106b by the bend mirror 104. One of the galvanometer scanners 106a and 106b is rotated in a direction corresponding to the X direction of the X-Y stage 109, and the other is rotated in a direction corresponding to the Y direction. Therefore, the laser beam 102 can be scanned at an arbitrary position within a limited area on the X-Y plane. The laser beam 102 is incident on the Fθ lens 108 at various angles. The laser beam 102 is corrected such that the laser beam 102 is incident on the Fθ lens 108 by the optical characteristics of the Fθ lens 108 perpendicularly to the X-Y stage 109.
In this manner, the laser beam 102 can be freely positioned by the galvanometer scanners 106a and 106b with respect to any coordinates on the X-Y plane within a limited area (to be referred to as a scan area) on the X-Y stage 109. The laser beam 102 is irradiated on the position to machine the object 107.
Upon completion of the machining in the scan area, the X-Y stage 109 moves to a position serving as a new scan area of the object 107 to repeat machining.
In particular, when the object 107 is a printed circuit board, and when it is desired to perform machining for a relatively precise hole, an optical system may be an image transfer optical system. FIG. 10 is a schematic diagram showing the positional relationships between the optical components when an image transfer system is used. In FIG. 10, reference symbol a denotes a distance between an aperture 110 for setting a beam spot diameter on the object 107 and the Fθ lens 108 on the optical path, reference symbol b denotes a distance between the Fθ lens 108 and the object 107 on the optical path, and reference symbol f denotes a focal distance of the Fθ lens 108. The focal distance f of the Fθ lens 108 is set to be equal to the distance between the Fθ lens 108 and a center position 111 on the optical path between the two galvanometer mirrors 105a and 105b. 
In the image transfer optical system the above positional relationships, the effective radiuses of the galvanometer mirrors 105a and 105b are represented by gr. In this case, when the distance a is sufficiently larger than the distance b, a numerical aperture NA in the optical system of the Fθ lens 108 and the object 107 is expressed by equation (1):NA=gr/(b2+gr2)1/2  (1)
When the wavelength of the laser beam is represented by λ, a beam spot diameter d on the object is expressed by equation (2)
 d=0.82λ/NA  (2)
In addition, since the image transfer optical system is used, a, b, and f are set to have such a positional relationship that the relations expressed by equation (3) is established.1/a+1/b=1/f  (3)
Therefore, for example, in order to obtain a beam spot diameter d of 95 μm by a laser having a wavelength λ of 9.3 μm, the numerical aperture NA must be 0.08 according to equation (2). In this manner, according to equation (2), in order to decrease the beam spot diameter d to perform precise hole drilling, the numerical aperture NA must be large.
For this purpose, it is understood according to equation (1) that the effective radius gr at which a laser beam from the galvanometer mirror can be reflected without deteriorating the quality of the laser beam is preferably increased. For example, in order to achieve a beam spot having a diameter at least smaller than the beam spot diameter d=95 μm by an optical system which satisfies f=100 mm and b=107 mm, b=107 mm is satisfied according to equation (3). For this reason, in order to satisfy NA>0.08, it is understood according to equation (1) that gr>8.6 mm is satisfied.
In order to improve the productivity of the laser machining device, the drive speed of the galvanometer scanner must be high. For this reason, in general, it is said that to decrease a galvanometer mirror or to decrease the deviation angle of the galvanometer mirror is effective.
Japanese Unexamined Patent Publication No. 11-192571 discloses a laser machining device which branches a laser beam with a branching means, guides respective laser beams to a machining position with scanning means, and converges the respective laser beam to perform machining.
In addition, Japanese Unexamined Patent Publication No. 11-314188 discloses a laser machining device in which a laser beam is split by a half mirror, and split laser beams are guided to a plurality of galvanometer scanners and irradiated on a plurality of machining areas through Fθ lenses.
However, when a galvanometer mirror diameter is decreased, an effective radius gr decreases, and a numerical aperture NA decreases according to equation (1). As a result, a beam spot diameter d which satisfies the relation expressed equation (2) increases, and such a problem that precise hole drilling cannot be performed is posed.
In addition, when the deviation angle of the galvanometer mirror is reduced, respective scan area sizes become small. For this reason, the number of scan areas increases. In general, since a time required for positioning by the galvanometer scanner 106 is considerably longer than a time required for positioning of the X-Y table, the number of scan areas increases. When the number of times of movement by the X-Y stage increases, although the speeds in the respective scan areas increases, such a problem that the entire production rate is not improved is posed.
Furthermore, in the device disclosed in Japanese Unexamined Patent Publication No. 11-192571, in order to control and converge slit laser beams, galvanometer scanners (galvanometer meters and galvanometer mirrors) and Fθ lenses corresponding to the respective laser beams are required. For this reason, when a laser beam is split into two laser beams, galvanometer scanners and Fθ lenses the numbers of which are twice the numbers of galvanometer scanners and Fθ lenses of the laser machining device shown in FIG. 9, and the problem of an increase in cost is posed. In order to simultaneously machine two objects to obtain twice machining speeds, an X-Y table the size of which is twice the size of the X-Y table of the laser machining device is required, and such a problem that the machining device increases in size is posed.
Still furthermore, in Japanese Unexamined Patent Publication No. 11-314188, respective split laser beams are guided to a plurality of independent galvanometer scanner systems and converged by Fθ lenses. For this reason, since a laser beam which is incident from the final galvanometer mirror onto the Fθ lens in the optical path is largely obliquely incident, the influence of the aberration of the Fθ lens increases, and such a problem that the laser beam cannot be easily converged in a small area.